The manufacture of electric vehicles powered by chemical batteries is being encouraged by air quality control agencies in an effort to reduce the air pollution created by the internal combustion engines in current use. Even though the electric power utilities which supply the energy used to charge the batteries are themselves polluters, the net result is favorable with respect to air quality. However, the relatively poor characteristics of chemical batteries, in terms of weight, cycle life, and cost make it difficult for them to compete in the marketplace with internal-combustion engines as the power system of choice.
A hybrid electric power train, consisting of a turbogenerator which generates the average power consumed by the vehicle, a flywheel surge power generator, an electric traction motor, and an electronic power control system can achieve the low pollution levels needed for good air quality, but with performance characteristics which exceed those of the internal combustion engine. Even though the turbine burns hydrocarbon fuels, its use of a catalytic combustor results in less air pollution than that created by the utilities which provide the electricity needed to charge the chemical batteries in vehicles so powered. The separation of the power sources into elements separately optimized to supply the average and the peak power, respectively, coupled with the ability to use dynamic braking, causes the efficiency over most driving schedules to be enhanced and, thus, less fuel is consumed.
A description of a turbogenerator suitable for use in a hybrid electric vehicle is given in a paper by Robin Mackay for the SAE International Congress and Exposition, March, 1994, entitled "Development of a 24-kW Gas Turbine Generator Set for Hybrid Vehicles," which paper is incorporated herein by reference for all purposes. Many different types of electric motors have been used for traction of electrically propelled vehicles for over a century. The present disclosure relates to the design of the flywheel energy storage system. The electric power control system, the fourth major element of the electric power train, is described in a U.S. Pat. No. 5,568,023, which is entitled "ELECTRIC POWER TRAIN CONTROL" and which is incorporated herein for all purposes.
Modem high strength-to-weight ratio fibers make it possible to construct high energy density flywheels, which, when combined with a high power motor-generators, are an attractive alternative to electrochemical batteries for use as energy buffers in hybrid electric vehicles. A properly designed flywheel system would provide higher energy density, higher power density, higher efficiency, and longer life than a conventional electrochemical battery.
The vehicle environment, however, presents special challenges to successful implementation of a flywheel to motor vehicle applications. Among these challenges are the need to deal with the gyroscopic torques resulting from the vehicle's angular motions and the need to accommodate the translational accelerations of the vehicle. Several safety issues resulting from the high energy and momentum stored in the flywheel also need to be taken into account, as does the difficulty of cooling the motor-generator operating in a vacuum chamber. In addition, energy conservation considerations and user convenience dictate the requirement that the flywheel storage system possess a slow self-discharge rate.
Flywheel energy storage systems have been proposed for many years; many of the storage systems have even been proposed for use in motor vehicles. U.S. Pat. No. 3,741,034, for example, discloses a flywheel contained in an evacuated sphere which is surrounded by a liquid and having various safety features. However, the '034 patent does not address waste heat production and the requirement for cooling the motor-generator. In addition, the '034 patent does not address itself to the dynamics of the driving environment, or the minimization of the power drain when parked. U.S. Pat. Nos. 4,266,442, 4,285,251 and 4,860,611, on the other hand, disclose different ways of constructing high speed rotors. However, the above referenced patents do not recognize, let alone describe, design features needed for compatibility with the environment of a motor vehicle.
Moreover, in order to accommodate a rim speed of about about 1000 meters per second, a housing containing the flywheel should be maintained at a very low pressure, e.g., a pressure below 0.01 Pascal, to limit windage losses. While this pressure can be readily achieved before sealing the housing, the fiber composite materials used in the construction of high energy density flywheels have a residual gas evolution rate which make it difficult to achieve this desired degree of pressure, i.e., near vacuum conditions, in a sealed container. Thus, continuous pumping of the evolving gases from the container is often needed. Most often, an external pump is employed to maintain the desired pressure.
U.S. Pat. Nos. 4,023,920, 4,732,529 and 4,826,393 describe various implementations of molecular pumps, which are a class of high vacuum pump wherein the dimensions of the critical elements are comparable to the mean free path of the gas molecules at the pressure of interest. Two types are generally known, a turbo-molecular pump, which is similar in construction to an axial flow compressor in a gas turbine employing interleaved rotor and stator blades, and a molecular drag pump, which uses helical grooves cut in the stator, which, in turn, is disposed in close proximity to a high speed rotor so as to direct gas flow through the pump. It will be appreciated that hybrid molecular pumps, which pumps contains separate sections of each of these types or molecular pumps, are also known. More specifically, U.S. Pat. No. 4,023,920 discloses a turbo-molecular pump using magnetic bearings to support the pump rotor at high rotational speeds. U.S. Pat. Nos. 4,732,529 and 4,826,393 disclose hybrid molecular pumps in which a turbo-molecular section is used on the high vacuum input side and a spiral groove drag pump is used on the discharge side.
All of these pumps are designed as self-contained systems, each with its own shaft, bearing system and power source, i.e., motor. While this solution is satisfactory for stationary systems, it is more difficult to apply in mobile applications because the space and weight for its implementation is not readily available.
As discussed above, flywheel systems currently being designed for mobile energy storage are generally intended to replace batteries in electrically powered vehicles. In such applications, multiple units are needed to store the required energy, so that each motor-generator need supply only a small portion of the vehicle's power. In systems where all of the surge power must be supplied by a single flywheel, the relatively large size of the single motor-generator makes it difficult to provide the needed energy density without reducing safety factors, e.g., for radial stresses, to unacceptable low levels or raising manufacturing costs to exorbitantly high levels.
The above-mentioned U.S. Pat. No. 3,741,034 discloses rotor designs using high strength-to-weight ratio filament wound composites in relatively thin concentric cylinders, which cylinders are separated by radial springs. While this arrangement limits the radial stresses to tolerable values, it is expensive to manufacture. U.S. Patent No. 3,859,868 discloses techniques for varying the elasticity-density ratio of the rotor elements to minimize radial stresses. On the other hand, U.S. Pat. Nos. 4,341,001 and 4,821,599 describe the use of curved metallic hubs to connect the energy storage elements to the axle. Additionally, U.S. Pat. No. 5,124,605 discloses a flywheel system employing counter-rotating flywheels, each of which includes a hub, a rim and a plurality of tubular assemblies disposed parallel to the hub axis for connecting the hub to the rim while allowing for differential radial expansion between the hub and the rim.
None of the latter references deal with the integration of a large, high power motor-generator into the flywheel energy storage system currently being designed for vehicles.
The present invention was, thus, motivated by a desire to provide an improved flywheel-motor-generator energy storage system suitable for moving vehicles. More specifically, the present invention was motivated by a desire to correct the perceived weaknesses and identified problems associated with conventional flywheel energy storage systems.